INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY

CHEMISTRY AND THE ENVIRONMENT DIVISION

COMMISSION ON ATMOSPHERIC CHEMISTRY^*

(Revision, 2000)

At its 1999 meeting at the IUPAC General Assembly in Berlin the Commission on Atmospheric Chemistry agreed to undertake a revision of this document. It was agreed to place the earlier version on the web and to invite comment from the interested atmospheric chemistry community. The project of revising the document will be headed up by Stephen E. Schwartz of Brookhaven National Laboratory.

> see project 1999-033-1-600

The document on this page is a html version of the published glossary. At present it still contains numerous errors of formatting of formulas and of symbols, which will be corrected.

Visitors to this page are invited to send comments to ses@bnl.gov with subject line "IUPAC Glossary".

Are there definitions that are unnecessary? For example Hydrocarbons. Chemists should know what these are; a full definition would be daunting; and there is a lot of non IUPAC nomenclature in the existing definition.

Are there items to be added? Please suggest a definition.

Are there items to be modified? Please indicate changes.

It is intended to set up a web page on which suggestions will be registered and can in turn be commented on by others.

Attention is called to the fact that the IUPAC Gold Book of Chemical Nomenclature, referred to in the 1990 Glossary, is now available on line:

The Gold Book

Compendium of Chemical Terminology
Gold, V.; Loening, K.L.; McNaught, A.D. and Shemi, P.
Blackwell Science, 1987 [ISBN 0-63201-7651(8)]

Compendium of Chemical Terminology, 2nd edition
McNaught, A.D. and Wilkinson, A.
Blackwell Science, 1997 [ISBN 0-86542-6848]

We welcome your suggestions. Thank you for your interest and help in this project.

Steve Schwartz 2000-06-05.

Pure Appl. Chem., Vol. 62, No. 11, pp. 2167-2219, 1990.

©1990 IUPAC

INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY

APPLIED CHEMISTRY DIVISION

COMMISSION ON ATMOSPHERIC CHEMISTRY^*

(Recommendations 1990)

JACK G. CALVERT

Atmospheric Chemistry Division

National Center for Atmospheric Research, Box 3000, Boulder, CO 80307, USA

^*Membership of the Commission during the period 1986-1990 in which the report was prepared was as follows:

Chairman: 1985-1987 K. Leichnitz (FRG;); 1987-1990 J. G. Calvert (USA); Secretary: 1986-1987 C. J. Purnell (UK): 1987-1990 J. Slanina (Netherlands); Titular members: H. Akimoto (Japan; 1987-1990); S. A. Penkett (UK; 1987-1990); H. R. Pruppacher (FRG; 1986-1987); H. I. Schiff (Canada; 1986-1990); X. Tang (China; 1987-1990); P. Warneck (FRG; 1988-1990); Associate Members: N. M. Bazhin (USSR; 1987-1990): R. H. Brown (UK; 1987-1990); K. H. Eickel (FRG; 1989-1990); J. A. Kerr (Switzerland; 1986-1990); D. Klockow (FRG; 1987-1989); I. O'Neil (France; 1986-1989); M. Sauté (France; 1986-1989); P. Tevault (USA; 1987-1989); P. Warneck (FRG; 1986-1988); W. E. Wilson (USA; 1989-1990); National Representatives: R. A. Cox (UK; 1986-1990); K. H. Eickel (FRG; 1986-1989); H.-J. Grosse (GDR; 1986-1990); L. Klasinc (Yugoslavia; 1989-1990); Ø. Hov (Norway; 1986-1990); J.-E. O. Lindqvist (Sweden; 1986-1990); U. Özer (Turkey; 1986-1990); L. F. Phillips (New Zealand; 1986-1990); O. N. Singh (India; 1989-1990).

Over 620 atmospheric chemistry terms which are in common use by the international research community in the atmospheric sciences are defined. The diversified nature of the scientists involved in research in atmospheric chemistry (chemists, meteorologists, physicists, biologists, engineers, and others) requires that special attention be paid to the consistency of the terminology employed. Where alternative terms are in common use to describe a given process, piece of equipment, feature, or quantity used in atmospheric chemistry, recommended choices of the IUPAC Commission on Atmospheric Chemistry are given; these are consistent with the other IUPAC publications and recommendations on nomenclature. The range of terms includes composition of the atmosphere, clouds, solar radiation, chemical and physical changes in atmospheric components, instrumentation employed in analysis of the atmospheric components and in air quality control, and in effects of certain trace gases on materials, plants, and animals.

The present document was prepared as part of a project of the IUPAC Commission on Atmospheric Chemistry. It was the judgment of this Commission that the unambiguous definition of terms and units employed by the international research community in atmospheric chemistry is essential to the achievement of reliable communications between ail parties. The diversified nature of the scientists involved in research in atmospheric chemistry, chemists, meteorologists, physicists, biologists, engineers, and others, points to the very special need for a definition of the common terms employed by each specialist with recommendations where alternative terms or units are in use today. In addition to the definition of terms, some examples are given in many cases to illustrate the use of the specific term in atmospheric chemistry. It is hoped that the reader will be tolerant of the inclusion in this Glossary of some terms which are obvious to the specialist in the area of their use, but are unclear to many others who work actively in other areas associated with atmospheric chemistry. In order to enhance the convenience of this Glossary to the user and in view of the specialized nature of many of the atmospheric chemistry terms, we have repeated here the definitions of many terms which appear in other IUPAC publications, rather than including only a reference to the IUPAC publication. All terms taken from the IUPAC Compendium of Chemical Terminology, Blackwell Scientific Publications, 1987, commonly referred to as the "Gold Book", are preceded by the designation GB (Gold Book) in this Glossary. In cases where the definition given here is different from that given by some other IUPAC publication, we have taken care to ensure the compatibility of the definitions while including portions of the definition which are peculiar to atmospheric chemistry.

It is the hope of this Commission that the Glossary will serve the Atmospheric Chemistry research community and lead to a more precise and uniform choice of common terms in the communication between atmospheric scientists around the world.

In the preparation of this document, the members of the Commission relied not only on their own experience and knowledge, but we are grateful to our many colleagues in the field of atmospheric chemistry and the members of the related IUPAC Commissions (Chemical Kinetics: I.4; Molecular Structure and Spectroscopy: I.5; Photochemistry: III.3: Analytical Nomenclature: V.3), IUPAC Divisions (Physical Chemistry: I; Inorganic Chemistry: II; Organic Chemistry: III; Macromolecular: IV; Analytical Chemistry: V; Applied Chemistry: VI; Clinical Chemistry: VII), and the Interdivisional Committee on Nomenclature and Symbols who reviewed the material and provided helpful suggestions. We also made extensive use of several sources of information: the IUPAC Compendium of chemical terminology (1987); the IUPAC book on Quantities, units und symbols in physical chemistry (1988); Air quality --Performance characteristics and related concepts for air quality measuring methods, International Standard, ISO 6879-1983 (E); Air quality--General aspects--Units of measurement, International Standard, ISO 4226-1980 (E); Gas analysis--Vocabulary, First edition, 1984, International Standard, ISO 7504; Air quality·--General Aspects-- Vocabulary, First edition 1980, International Standard, ISO 4225; Standard definitions of terms relating to atmospheric sampling and analysis, ANSI/ASTM D 1356-73A.

Abatement: Action taken to reduce air pollution which involves the use of control equipment or some new process. This refers to a reduction or lessening as opposed to elimination of a type of discharge or pollutant.

Absorbance: The logarithm (must be specified as to base 10, lg, or base e, ln) of the reciprocal of transmittance [ln(I_o/I_t) or lg(I_o/I_t)], where I_o and I_t are the monochromatic radiances (intensities) of light incident on and transmitted through, respectively, a sample which is usually contained in a sample cell.

Absorber: A device used commonly for sampling by absorption in which a gaseous or liquid material is removed from another gas or liquid by selective absorption; these include: scrubber, impinger, packed column, spray chamber, etc. GB A substance used to absorb energy form any type of radiation.

Absorptance, absorption-factor GB The ratio of absorbed to incident radiant or luminous flux.

Absorption: The process of one material (absorbent) being retained by another (absorbate); this may be the physical solution of a gas, liquid, or solid in a liquid, attachment of molecules of a gas, vapour, liquid, or dissolved substance to a solid surface by physical forces, etc. In spectrophotometry, absorption of light at characteristic wavelengths or bands of wavelengths is used to identify the chemical nature of molecules, atoms or ions and to measure the concentrations of these species. GB: The transfer of a component from one phase to another.

Absorption coefficient: Quantity used to specify the absorption of light at a given wavelength by a pure compound; it is the constant e which appears in the Beer-Lambert law: lg(I_o/I_t) = [A]le , where I_o and I_t are the incident and transmitted monochromatic radiances (intensities), respectively for an optical path of length l and uniform concentration of absorbing compound [A] (commonly mol L^-1, coherent SI units; mol m^-3). The coefficient e is sometimes called the (decadic) molar absorption coefficient. Although it is sometimes referred to as the extinction coefficient, this is not a good practice since extinction strictly implies the sum of absorption and scattering. There are several useful forms of this relation which are used in describing light absorption. When napierian (natural) logarithms are used, the symbol k is recommended with the name napierian molar absorption coefficient or molar cross section. Pressure of a gas (STP) may be used rather than concentration, etc. See also absorption cross section. The specific form of the law employed (base of the logarithms. concentration, and pathlength units) must be stated with absorption coefficient data derived from it if the data are to be of use to the scientific community.

Absorption cross section: A constant (s ) in the following relation describing the absorption of light of a given wavelength: ln(I_o/I_t) = [A] ls ; here I_o and I_t are the monochromatic radiances (intensities) of the incident and transmitted light, [A] is the concentration (molecules cm^-3), and l is the path length (cm); s (units of cm^-2/molecule commonly) is related to the molar cross section k by: s = k /N_A where N_A is the Avogadro constant (6.02213 x 10²³ mol^-1).

Absorption line: A narrow range of wavelengths in which a substance absorbs light; a series of discrete absorption lines can be used as an unambiguous identification for many relatively simple chemical species.

Absorption spectrum: The wavelength dependence of the absorption cross section (or absorption coefficient); usually represented as a plot of absorption cross section versus wavelength l (or 1/l ) of the light.

Accommodation coefficient: Also sticking coefficient. A measure of the efficiency of capture of molecules or atoms which collide with aerosol particles, cloud droplets, etc. The accommodation coefficient is the fraction of the collisions which result in the capture of the molecules (atoms, radicals, etc.) by the particle, cloud droplet, etc.,; fraction of colliding molecules which are not reflected but which enter the surface of an aqueous aerosol.

Accretion: The process by which aerosols grow in size by external addition of various chemical species; a form of agglomeration.

Accuracy: The closeness of agreement between the true value and the measured value. The definition given by the ISO International Standard for reference: Agreement between a single measured value,
_a, and the value of air quality characteristic, C_a, itself, or the accepted reference value; accuracy denotes the extent to which bias and random errors are absent.

Acid: A species which reacts in liquid water to generate hydrogen ions (conventionally represented as cations H⁺ or hydronium, H₃O⁺); anions (e.g., sulphate, SO₄^2-; nitrate, NO₃^-) which were associated with the H⁺ in the acid are also released. Important acids in the atmosphere include sulfuric acid (H₂SO₄), nitric acid (HNO₃), and organic acids (e.g., formic acid, HCO₂H; acetic acid, CH₃CO₂H, etc.). GB: Brønsted acid: A molecular entity capable of donating a proton to a base, (i.e. a "proton donor") or the corresponding chemical species. For example: H₂O, H₃O⁺, CH₃CO₂H, H₂SO₄, HSO₄^-. GB: Lewis acid: A molecular entity (and the corresponding chemical species) that is an electron-pair acceptor and therefore able to react with a Lewis base to form a Lewis adduct, by sharing the electron pair furnished by the Lewis base. For example:

Me₃B + :NH₃ --> Me₃B^- - ⁺NH₃

Lewis acid + Lewis base --> Lewis adduct

GB: Hard acid: A Lewis acid with an acceptor centre of low polarizability. It preferentially associates with hard bases rather than with soft bases, in a qualitative sense. For example, the hard O- (or N-) bases are preferred to their S- (or P-) analogues. Conversely a soft acid possesses an acceptor centre of high polarizability and exhibits the reverse preference of a partner for co-ordination. These preferences are not defined in a quantitative sense.

Acid deposition: The acids deposited by transfer from the atmosphere either by precipitation (rain, fog, snow, etc.), called wet deposition, or by direct transfer to the surface of the earth induced by the flow of acid-containing air masses over the earth's surface, called dry deposition.

Acid rain: Rain with pH values < about 5; commonly results from acids formed from pollutants. "Pure" rain water equilibrated with atmospheric CO₂ and naturally occurring acids in relatively clean air usually has a pH > 5.

Actinic flux (Sl ): The quantity of light available to molecules at a particular point in the atmosphere and which, on absorption, drives photochemical processes in the atmosphere. It is calculated by integrating the spectral radiance L(l ,q ,f ) over all directions of incidence of the light, ºq ºf { L(l ,q ,f )/(hc/l ))} sinq dq df . If the radiance is expressed in J m^-2 s^-1 st^-1 nm^-1 and hc/l is the energy per quantum of light of wavelength l , the actinic flux has units of quanta cm^-2 s^-1 nm^-1. This important quantity is one of the terms required in the calculation of j-values, the first order rate coefficients for photochemical processes in the sunlight-absorbing, trace gases in the atmosphere. The actinic flux is determined by the solar radiation entering the atmosphere and by any changes in this due to atmospheric gases and particles (e.g., Rayleigh scattering, absorption by stratospheric ozone, scattering and absorption by aerosols and clouds), and reflections from the ground. It is therefore dependent on the wavelength of the light, on the altitude, and on specific local environmental conditions. The actinic flux has borne many names (e.g., flux, flux density, beam irradiance, actinic irradiance, integrated intensity) which has caused some confusion. It is important to distinguish the actinic flux from the spectral irradiance, which refers to energy arrival on a flat surface having fixed spatial orientation (J m^-2 s^-1 nm^-1) given by E(l ) = ºq ºf L(l ,q ,f ) cosq sinq df df . The actinic flux does not refer to any specific orientation because molecules are oriented randomly in the atmosphere. This distinction is of practical relevance: the actinic flux (and therefore a j-value) near a brightly reflecting surface (e.g., over snow, or above a thick cloud) can be a factor of three higher than that near a non-reflecting surface. The more descriptive name of spectral spheradiance is suggested for the quantity herein called actinic flux. See also flux density, photon.

Activity, absolute: (GB) The absolute activity l _B of a substance B in a mixture of substances B, C,... is a number defined by

l _B = exp(m _B/RT) or m _B = RTlnl _B

where m _B is the chemical potential of the substance B and R is the gas constant.

Activity, relative: (GB) Of a solute substance B in a solution (especially in a dilute liquid solution) containing molalities m_B, m_C,..., of solute substances B, C,..., in a solvent substance A, a_B is a number defined in terms of the absolute activity l _B by

a_B = (l _B/m^ø)/(l _B/m_B)¥ = m_{Bg B}/m^ø (T, p constant)

or in terms of the chemical potential m _B by

RTlna_B = m _B — RTlnm^ø — (m _B — RTlnm_B)¥

= RTln(m_{Bg B}/m^ø)

where m^ø is a standard value of molality (usually chosen to be 1 mol kg^-1), g _B is the activity coefficient of B, R is the gas constant, T is the thermodynamic temperature, and p is the pressure. The superscript ¥ denotes infinite dilution.

Activity coefficient: The coefficient (g ) measures the degree of departure of a component in solution from ideal behavior. It relates concentration [ A] and activity (a) for a substance in solution: a = g [A]. The concentrations and activities of components are often given in molal units (mol kg^-1 of solute). The activity coefficient, g , is usually determined empirically; it depends not only upon the concentration of the substance but on its particular properties and on the concentration and kind of other substances present. As the concentration of the solute approaches zero, the activity coefficient approaches unity. GB: The activity coefficient g _B of a single substance B in a solution (especially in a dilute liquid solution) containing molalities m_B, m_C,..., of solute substances B, C,..., in a solvent substance A, is a number defined in terms of the absolute activity g _B of the solute substance B in the solution by

g _B = (l _B/m_B)/(l _B/m_B)¥ (T, p constant)

or in terms of the chemical potential m _B by

RTln(m_{Bg B}) = m _B — (m _B — RTlnm_B)¥ (T, p constant)

where the superscript ¥ denotes infinite dilution. The activity coefficient ƒ_B of a substance B in a liquid or solid mixture containing mole fractions x_B, x_C,…, of the substances B, C,..., is a number defined in terms of the absolute activity l _B of the substance B in the mixture by

ƒ_B = l _B/l _B^*x_B

where l _B^* is the absolute activity of the pure substance B at the same temperature and pressure, or in terms of the chemical potential m _B by

RTln(x_Bƒ_B) = m _B — m _B^*

and m _B^* is the chemical potential of the pure substance B at the same temperature and pressure.

Adiabatic lapse rate: The rate of decrease in temperature with increase in altitude of an air parcel which is expanding slowly to a lower atmospheric pressure without exchange of heat; for a descending parcel it is the rate of increase in temperature with decrease in altitude. Theory predicts that for dry air it is equal to the acceleration of gravity divided by the specific heat of dry air at constant pressure (approximately 9.8 °C/km). The moist adiabatic lapse rate is less than the dry adiabatic lapse rate and depends on the moisture content of the air mass.

Adsorbate: A molecular species of gas, dissolved substance or liquid which adheres to or is adsorbed in an extremely thin surface layer of a solid substance. GB: The material accumulated on the surface of an adsorbent by the process of adsorption.

Adsorbent: An adsorbent is the solid material to which another species is adsorbed in a surface layer. GB: A condensed phase at the surface of which adsorption may occur.

Adrsorber: Solid material used to capture either a gas or liquid; often activated carbon is employed as the solid because of its high surface area per unit mass.

Adsorption: The process by which molecules of gas, dissolved substances, or liquids adhere to the surface of solids through either weak physical forces (physical adsorption) or stronger chemical forces (chemical adsorption). GB: The enrichment (positive adsorption, or briefly, adsorption) of one or more components in an interfacial layer.

Advection: The transport of air, its properties (such as heat), trace materials, fog, cold air, etc., solely by mass motion of the atmosphere, generally in a horizontal direction.

Aeration: The process by which a volume filled with a liquid becomes permeated with air or another gas; aeration is often accomplished by spraying the liquid into the air, bubbling air through a liquid, or agitating the liquid to promote surface absorption of air.

Aerometer: Instrument used to measure the density of gases.

Aerometric measurements: Measurements of the temperature, pressure, air motion (velocity vectors), or other physical measurements to characterize an air mass.

Aerosol: Mixtures of small particles (solid, liquid, or a mixed variety) and the carrier gas (usually air); owing to their size, these particles (usually less than 100 m m and greater than 0.01 m m in diameter) have a comparatively small settling velocity and hence exhibit some degree of stability in the earth's gravitational field. An aerosol may be characterized by its chemical composition, its radioactivity, the particle size distribution, the electrical charge and the optical properties. GB: Aerosols are dispersions in gases. In aerosols the particles often exceed the usual size limits for colloids. If the dispersed particles are solid, one speaks of "aerosols of solid particles", if they are liquid of "aerosols of liquid particles". The use of the terms "solid aerosol and "liquid aerosol" is discouraged. An aerosol is neither "solid" nor "liquid", but if anything, gaseous.

Agglomeration: The growth of particles in size due to an attachment of other particles following collisions. The term refers to aerosol particles, cloud droplets, etc.

Air, composition of pure: The composition of air is variable with respect to several of its components (e.g., CH₄, CO₂, H₂O) so "pure" air has no precise meaning; it is commonly considered to be air which is free of dust, aerosols, and reactive gaseous contaminants of anthropogenic origin. The composition of the major components in dry air is relatively constant (percent by volume given): nitrogen, 78.084; oxygen, 20.946; argon, 0.934; carbon dioxide, 0.033; neon, 0.0018; helium, 0.000524; methane, 0.00016, krypton 0.000114; hydrogen 0.00005; nitrous oxide, 0.00003; xenon, 0.0000087. The concentration of carbon dioxide, methane, nitrous oxide, the chlorofluorocarbons and some other species of anthropogenic origin are increasing measurably with time. Relative clean air which is free of most reactive anthropogenic pollution (NO, NO₂, SO₂, non-methane hydrocarbons, etc.), often used as a reference sample in the calibration and operation of instruments, is purchased under the designation of zero air.

Air mass: A qualitative term to describe a widespread body of air with approximately uniform characteristics which had been identified at a given time over a particular region of the earth's surface. Sometimes an air mass is marked by inert tracers such as SF₆ which may be added to it. The composition of a given air mass undergoes alteration as it migrates, chemical changes occur, compounds are removed by dry and wet deposition, and new impurities are added to the mass.

Air monitoring station: A site at which monitoring of the concentration of one or more pollutants is carried out (e.g., the BAPMoN stations. Background Air Pollution Monitoring Network of the WMO, World Meteorological Organization).

Air pollutant: A substance, gaseous material or aerosol which has been introduced into the air (either by human activity or by natural processes) in sufficient concentration to produce a measurable effect on humans, animals, vegetation, or materials (monuments, etc.): SO₂, NO₂, H₂S, CO, hydrocarbons, etc.

Air pollution: Usually the presence of substances in the atmosphere, resulting either from human activity or natural processes, present in sufficient concentration, for a sufficient time and under circumstances such as to interfere with comfort, health or welfare of persons or the environment.

Air pollution control: Measures taken to control the purity of the air to some degree specified by law.

Air pollution control district: A geographical region designated by law in which the emissions of some specified air pollutants are controlled to a degree specified by law.

Air pollution index or air quality index: A mathematical combination of the concentrations of air pollutants (weighted in some fashion to reflect the estimated health impact of the specific pollutant) which gives an approximate numerical measure of the quality of the air at a given time. These indices have little scientific basis but have been used to inform the public (in a qualitative fashion) of the degree of pollution present at a given time. It is recommended that the actual measured pollutant concentrations be used by all information services when possible with simultaneous reference given to the corresponding concentrations which are considered by health authorities to be hazardous to human health.

Air pollution survey: A study of the concentrations and geographical distribution of specified air pollutants in a given area and an assessment of the damage, if any, which the pollutants have caused.

Air quality characteristic: One of the quantifiable properties relating to an air sample: concentration of a constituent, wind speed, temperature, etc. The quantity of air quality characteristic is the true value of the characteristic being investigated; it is recognized that in practice, this value can only be approximated by existing methods.

Air resource management: The detailed planning and the implementation of air pollution control programs designed to preserve the health and welfare of the people in the region, the plant and animal life, physical property, good visibility, and other factors which determine the air quality and the maintenance of an aesthetically acceptable environment.

Air sampling network: A number of air sampling stations which are established in a given geographical region at which measurements of both pollutant concentrations and meteorological quantities (wind speed, direction, rain fall, humidity, etc.) are made to determine the extent and the nature of the air pollution and to establish trends in the concentrations of the air pollutants with time.

Aitken particles: Aerosol particles below 0.1 m m in diameter. These generally are the most numerous among all particles in the air. Their concentrations can be determined with the Aitken counter which measures total particle number density. Owing to their small size, Aitken particles contribute little to the total mass concentration of all aerosol particles; this is determined primarily by particles of diameter > 0.1 m m.

Albedo: The fraction of the energy of electromagnetic radiation reflected from a body (or surface) relative to the energy incident upon it. The reflection of light from a surface is, of course, dependent on the wavelength of the light, the nature of the surface, and its angle of incidence with the surface. The term albedo usually connotes a broad wavelength band (visible, ultraviolet, or infrared), whereas the terms reflectivity and spectral albedo are used to describe the reflection of monochromatic (single wavelength or small band of wavelengths) radiation.

Aldehyde: A class of organic compound of general formula: R(C=O)H, where R is an hydrogen atom or hydrocarbyl group (e.g., CH₃, C₂H₅, etc.). These are products of hydrocarbon oxidation. The simplest aldehydes are formaldehyde (methanal, CH₂O), acetaldehyde (ethanal, CH₃CHO), and propionaldehyde (propanal, CH₃CH₂CHO). Aldehydes absorb sunlight (l < 330 nm) and generate radicals. They also react rapidly with HO-radicals in the atmosphere to produce acyl (RCO) and acylperoxyl (RCOO₂) radicals. Formaldehyde is the most abundant aldehyde in the atmosphere (formed from the natural hydrocarbon, CH₄, as well as other hydrocarbons) where its mixing ratios are often > 0.2 ppbv even in relatively clean air. Acetaldehyde is a major precursor to peroxyacetyl nitrate (PAN) in the troposphere.

Alert levels: Designated concentrations of certain key pollutants at which some degree of danger to public health is expected. In many areas in which a relatively high level of pollution is often encountered, several levels of alert are often established. For example, a first alert may signify a potential problem exists; a second alert becomes a signal for the curtailment of certain significant emission sources; the third alert signifies the need for some specified emergency action which must be taken to ensure the public safety.

Aliquot: A representative portion of the whole.

Ambient air: The outdoor air in the particular location.

Ambient air quality: A general term used to describe the quality of the outside air. Usually adjectives such as good, fair, bad, etc. are used by the media to describe this; often some form of air pollution or air quality index is employed to determine the specific descriptive term to be used. These are very qualitative terms of little or no scientific value. See also air pollution index, air quality index.

Ames/salmonella test: A screening test employed in predicting the mutagenic and the potential carcinogenic activities of chemicals in the environment. It employs Ames test strains of salmonella bacteria (his^-) which lack the ability to produce histidine. The compound to be tested, the bacteria and a small amount of histidine (insufficient to permit colony growth but enough to allow sufficient growth for expression of mutations) are added to agar. The bacteria are allowed to incubate for about 63 hours at 37 °C. If a significant increase in colonies above background is observed in the sample containing the test compound, then it is concluded that the chemical tested is a direct mutagen for the particular Ames strain of bacteria. This is taken as a qualitative indication of the possible carcinogenic activity of this chemical in other biological systems. This procedure seems to be of qualitative value in a preliminary screening of potential carcinogens.

Analyser, accuracy of an analysis with: The ability of a measuring instrument to give indications approaching the true value of the quantity measured.

Analyser, continuous: An analyser in which subassemblies operate continuously.

Analyser, continuous indication: An analysis with a permanent indication related to the sample concentration. To obtain a continuous indication, the sampling and measuring cell need to be continuous. A time lag may exist between sampling and indication of measured concentration; see analyser, dead time of.

Analyser, dead time of: Time which elapses between the moment at which a sudden change in concentration (or a new sample) is introduced and the moment at which the detector response indication reaches the measurement threshold of the analyser, a value conventionally fixed at 10% of the final change in indication.

Analyser, discontinuous: An analyser with at least one discontinuous subassembly.

Analyser, discontinuous indication of: Indication related to the concentration during intervals of time which are not continuous.

Analyser, fall time of: When following a change in concentrations from a definite value c to 0, the time which elapses between the moment when this change is produced and the moment when the indication reaches a value conventionally fixed at 10% of the final change or indication.

Analyser, measurement threshold of: The minimum concentration of a substance which produces a quantifiable signal with a given uncertainty.

Analyser, precision of measurements with: The quality which characterizes the ability of a device to give for the same value of the quantity measured, indications which agree among themselves, not taking into consideration the systematic errors associated with variations of the indications. Repeatability characterizes the ability of a device to give indications which are unaffected by random errors.

Analyser, range of measurement of: Range of concentration between the measurement threshold and the maximum usable indication.

Analyser, resolution of: A general term which defines the ability of a device to differentiate between adjacent signals.

Analyser, response time of: Time which elapses, when there is a stepwise change in the quantity to be measured, between the moment when this change is produced and the moment when the indication reaches a value conventionally fixed at 90% of the final change in indication.

Analyser, rise time of: The difference between the response time and the dead time.

Analyser selectivity of analyses with: Ability of a device to exhibit a low or zero sensitivity to analytes (see interferant) other than the one it is intended to measure. The selectivity is described by a coefficient of interaction; it is the ratio of the device response to the substance to be measured to that of the interferant, both being at the same concentration; see Selectivity.

Analyser, sensitivity of: The response of the measuring cell of a system in indicated output units per unit concentration of the component being measured. It is the slope, at the concentration being measured, of the tangent to the calibration curve.

Analyser, sequential: A discontinuous analyser in which at least one subassembly operates sequentially.

Analyser sequential indication of: Indication obtained following sequential sampling or received from a sequential cell or from data processing comprising a succession of predetermined repetitive operations (or a combination of the three).

Analytical function: Function which relates the measured value
_a to the instrument reading, X, with the value of all interferants, C_i, remaining constant. This function is expressed by the following regression of the calibration results.

_a = ƒ(X) %

% C_i = constant

The analytical function is taken as equal to the inverse of the calibration function.

Analytical unit; analyser: An assembly of sub-units comprising: suitable apparatus permitting the introduction and removal of the gas, liquid or solid to be analysed and/or calibration materials; a measuring cell or other apparatus which, from the physical or chemical properties of the components of the material to be analysed, gives signals allowing their identification and/or measurement; signal processing devices (amplification, recording) or, if need be, data processing devices.

Aneroid barometer: An instrument for monitoring the atmospheric pressure in which no liquid is employed, but rather changes in pressure between the atmosphere and a closed vessel bend a diaphram which moves a pointer on a scale.

Anticyclone: A large system of winds that rotate about a centre of high atmospheric pressure, clockwise (viewed from above) in the northern hemisphere and counterclockwise in the southern hemisphere.

Appearance energy (appearance potential): Refers to ionization of a molecule or atom by electron collision or photon absorption. In mass spectrometry it has often been reported as the voltage which corresponds to the minimum energy of the electrons in the ionizing beam necessary for the production of a given fragment ion. In photoionization it is the minimum energy of the quantum of light which produces ionization of the absorbing molecule. GB: It is recommended that the term appearance energy replace the term appearance potential and that the energy should be stated in SI units.

Arrhenius equation: The equation, k = Ae^-Ea/RT, which describes the rate coefficient (k) for an elementary reaction in terms of a preexponential factor A, activation energy E_a, temperature T (K), and gas constant R.

Ash: The solid residue which remains after the combustion of a fuel such as coal. Ash consists largely of heat treated mineral matter, but it may contain some products of the incomplete combustion of the fuel as well.

Atmosphere (of the earth): The entire mass of air surrounding the earth which is composed largely of nitrogen, oxygen, water vapour, clouds (liquid or solid water), carbon dioxide, together with trace gases and aerosols; see air, composition of pure.

Atmosphere (unit of pressure measurement): A unit of pressure measurement which has been employed in describing the pressure of gases in storage tanks. It is roughly related to the average pressure of the atmosphere at sea level. One atmosphere = 1.01325 x 10⁵ N m^-2 (Pa) "a 1013.25 mbar "a 760 Torr. The use of the atm unit of pressure measurement is no longer recommended by international scientific bodies; it has been recommended that standard pressure be redefined as 10⁵ Pa.

Atomize: To subdivide a liquid into very small particles; methods include: impact with a jet of gas, use of a spinning disk generator, vibrating orifice generator, etc.

Arrester: Equipment designed to remove particles from a gaseous medium.

Aspirator: Any apparatus that produces a movement of a fluid by suction (e.g., a squeeze bulb, pump, Venturi, etc.)

Anthropogenic: Produced by human activities.

Background concentration (level): Synonymous with baseline concentration. The concentration of a given species in a pristine air mass in which anthropogenic impurities of a relatively short lifetime are not present. The background concentrations of relatively long-lived molecules, methane, carbon dioxide, halocarbons (CF₃Cl, CF₂Cl₂, etc.), and some other species continue to rise due to anthropogenic input, so the composition of background air is undergoing continual change. Background concentration of a given species is sometimes considered to be the concentration of that impurity in a given air mass when the contribution from anthropogenic sources under study is absent.

Baffle chamber: A chamber used in incinerator design to promote the settling of fly ash and coarse particulate matter by changing the direction and/or reducing the velocity of the gases produced by the combustion of the refuse.

Bag filter: A large bag constructed of a suitable fabric which is often tubular in shape, into which a particle-containing air stream flows. Modern bags are constructed of a fabric which is capable of collecting all but very fine particles in the gas stream. The efficiency of the removal of particles of various size ranges changes with the amount of particles captured by the filter and the filtering time. The bag operates on the same principle as the one on a household vacuum cleaner.

Baghouse: An installation which contains many bag biters in parallel so that the resistance to air flow in a large installation is not seriously increased by the addition of these controls.

Base: A species which when dissolved in water generates hydroxide (OH^-) ions or is capable of reacting with an acid to form a salt. GB: A Brønsted base is a molecular entity capable of accepting a proton from an acid (i.e. a "proton acceptor") or the corresponding chemical species. For example: OH^-, H₂O, CH₃CO₂^-, HSO₄^-, SO₄^2—. A Lewis base is a molecular entity (and the corresponding chemical species) that is able to provide a pair of electrons and thus capable of co-ordination to a Lewis acid, thereby producing a Lewis adduct.

Baseline in gas chromatography: Portion of a chromatogram corresponding to the signal delivered by the detector when, under normal conditions of operation, only the carrier gas passes through the detector. The slow desorption of strongly adsorbed substances previously passed through the column may contribute to this baseline position (column bleed). GB: The signal trace recorded as a function of time in the absence of analyte.

Beer-Lambert Law: An equation useful in the description of light absorption by a compound of concentration [A] (often in mol L^-1) and with an optical path length l (often in cm): lg(I_o/I_t) = [A]le ; here I_o and I_t represent the radiance (intensity) of the incident and the transmitted light, respectively, and e is the decadic molar absorption coefficient with common units of L mol^-1 cm^-1 coherent SI units mol^-1 m². For a collimated beam having a unique pathlength through the absorbing medium, this law applies to irradiance as well.

Bias error: Systematic deviation of the results of a measurement process from the true value of the air quality characteristic. This deviation cannot be detected by merely repeating the method several times under prescribed conditions. GB: Characterizes the systematic error in a given analytical procedure and is the (positive or negative) deviation of the mean analytical result from the (known or assumed) true value.

Bimodal distribution: The occurrence of two maxima in a frequency distribution.

Biosphere: That part of the globe that encompasses all forms of life on the earth. It extends from the ocean depths to a few thousand meters of altitude in the atmosphere, and includes life forms at the earth's surface, in soils, and constituents which exchange materials with atmospheres, oceans, or surfaces.

Bivane: A wind vane used in turbulence studies to obtain horizontal and vertical components of the wind vector.

Blank reading or blank value: Instrument reading for a zero sample. A reading originating from the matrix, reagents and any residual bias in the measurement instrument or process, but not from the analyte.

Blowdown: Hydrocarbons purged during refinery shutdowns and startups which should be piped to storage systems for safe venting, flaring, or recovery. This term also applies to the purging of water in boiler operation, and serves in the control of dissolved solids in the boiler water.

Bond dissociation enthalpy (called bond dissociation energy in the older literature): The standard enthalpy change (D_rH^ø) which accompanies the dissociation of a specific bond in a molecule; e.g., CH₄(g) --> H(g) + CH₃(g); a temperature of 298 K is implied unless another temperature is specified. This may be contrasted with the term bond energy which refers to the average energy required to break all equivalent bonds in a molecule: e.g., the C-H bond energy in CH₄ is equal to the D_rH^ø/4 where D_rH^ø refers to the enthalpy change for the overall reaction, CH₄(g) --> 4H(g) + C(g). GB: The enthalpy (per mole) required to break a given bond of some specific molecular entity by homolysis.

Boundary layer: That well-mixed region of the lower atmosphere in which the turbulence is maintained largely by convective buoyancy induced by the upward heat flux originating from the solar-heated surface of the earth. During the afternoon this often extends from 1 to 5 km in height. Surface boundary layer is that region of the lower atmosphere where the shearing stress is constant. It is separated by the Ekman layer from the free atmosphere, where the behavior of the atmosphere approaches that of an ideal fluid in approximate geostrophic equilibrium (horizontal coriolis force balances the horizontal pressure force at all points in the field).

Breeching: The passage or conduit through which the exhaust products of combustion are carried to the stack or chimney.

Breeze: A term sometimes used to describe very fine particles of coke. Also a meteorological term for a gentle wind.

Bridge solution (in pH measurement): (GB) Solution of high concentration of inert salt, preferably comprising cations and anions of equal mobility, optionally interposed between the reference electrode filling and both the test and standard solution, when the test solution and filling solution are chemically incompatible. This procedure introduces into the operational cell a second.liquid junction formed usually in a similar way to the first.

Brownian motion: The movement of particles in a colloidal system such as an aerosol caused by collision with the molecules in the fluid in which the particles are imbedded.

b_scat: See scattering cross section.

Bubbler: An apparatus used to absorb certain water soluble components in a gas stream for later analysis. Usually it involves the use of a glass fritted tube which forces the air into small bubbles of high surface area during operation.

Calibration component: A component of a calibration mixture, present in the gaseous or vapour state, quantitatively and qualitatively defined, and used directly for testing and for calibration.

Calibration function: Instrument reading, X, as a function of measurable properties of the air quality characteristic under investigation represented by the reference material, with all interferants, C_i remaining constant. This may be quantified by the following regression function of the calibration results:

X = g(C_a) %

% C_i = constant

A calibration function designated by X = C_as + constant, where the sensitivity s is constant, is designated a linear function.

If the sensitivity is not a constant, the calibration function is called non-linear. Non-linearity may be expressed by higher order regression coefficients.

Calibration gas mixture: A gas mixture of known composition, generally comprising one or more calibration components and a complimentary gas.

Calibration gas mixture, methods of preparation of: a) Gravimetric method: A method in which each component of a mixture is successively added :o a gas cylinder which is weighed first empty and then after each addition. The concentration of each component is expressed as a mass ratio or mole ratio. b) Manometric method: A method in which each component of a mixture is added successively to a gas cylinder previously emptied and where the pressure is measured after each addition. The mole ratio can only be calculated from the pressure data if the deviation from ideality of the particular system is known. c) Static volumetric method: A method in which a mixture is prepared by combining two or more gases, contained in two or more separate calibrated volumes, all at known temperatures and pressures. The volume ratio of a component in the resulting mixture may be calculated from the known ratios of the calibrated volumes. The calculated volume ratio may differ from the mole ratio if the mixture does not behave ideally. d) Dynamic volumetric method: A method in which two or more gas streams, flawing at known volume flow rates under specified conditions are combined into a single stream. The volume ratio of any component in the resulting mixture is calculated from the ratio of the volume flow rates. Deviations from ideality of the mixture must be known in order to calculate the mole ratio unless the flow rates are measured in mass flow per unit time for all gases; in this case the mass or mole ratio can be directly calculated. e) Saturation method: A method in which a stream of gas is passed over or through a substance capable of evaporation or sublimation held at a fixed temperature. At equilibrium the concentration of the substance in the stream of gas will be determined by the vapour pressure of the substance at the temperature given. f) Permeation method: A method in which a stream of gas is passed over a porous or permeable tube or vessel containing a volatile substance usually present as the liquid and saturated vapour phases. The permeation rate of the substance through the walls of the tube should be constant at any given temperature as long as sufficient liquid and its vapour remain in the tube. The concentration of the substance in the stream of gas is determined by the rate of flow of the gas stream and the permeation rate of the substance. If the gas flow is measured in terms of mass per unit time and the permeation rate is known in the same units, then the mass ratio or the mole ratio can be calculated. g) Diffusion method: A method in which a gas or vapour diffuses through a restriction into a stream of gas flowing at constant rate. If the temperature and pressure of the diffusing species remain constant at the inlet to and within the restriction, the mass rate of now of the diffusing species will remain constant. h) Comparison method: A method in which the concentration of a component in a mixture is measured by comparison with another mixture or other mixtures containing the same component at known concentrations.

Calibration mixture: A gaseous or liquid mixture of known composition, generally comprising one or more calibration components and an inert diluent, used directly for testing and calibration of analytical instruments.

Carbon black: Finely divided forms of carbon made by the incomplete combustion or thermal decomposition of natural gas, higher molecular weight hydrocarbons or other carbon-containing compounds.

Carbon dioxide (CO₂): Colorless, odorless gas which is the major carbon-containing product of combustion of carbon compounds. Its removal in the atmosphere involves the photosynthetic cycle in plants which convert CO₂ into carbohydrate and other organic molecules important in plant growth, as oxygen is released. It is also removed by the oceans where it is converted to carbonate deposits. Its concentration in the atmosphere varies with position on the earth, season, time of day, etc. However its average mixing ratio in pristine air has increased from about 315 to 340 ppmv during the last 25 years, presumably as a result of increased burning of fossil fuels. Concern has been expressed in the possible effects of this increase on global climate change; see greenhouse effect.

Carbon monoxide (CO): Colorless, odorless gas which is highly poisonous even at the level of 0.2% in air. It is a product of the incomplete combustion of hydrocarbon fuels, and it is also formed in the clean atmosphere as a result of the oxidation of methane and its oxidation product formaldehyde (CH₂O); mixing ratios of CO in pristine air masses are usually of the order of 100 ppbv near sea level.

Carrier gas: A gas introduced in order to transport a sample for analytical purposes. In gas chromatography it is the gas which is passed continuously through the column and whose passage promotes the elution of the components of the sample. GB: The carrier gas together with the portions of the sample present in this phase constitutes the mobile phase.

Catalysis: The alteration of the rate of a chemical reaction (usually increase in the rate) by the addition of some substance (the catalyst) which does not undergo a net chemical change. GB: Cases occur with certain reactants in which the addition of a substance reduces the rate of a particular reaction, for example, the addition of an inhibitor in a chain reaction or a poison in a catalytic reaction. The term "negative catalysis" has been used for these phenomena, but this usage is not recommended; terms such as inhibition or poisoning are preferred.

Ceilometer: An automatic, recording instrument for reading the height of the cloud-base.

Cell, continuous measuring: A measuring cell which operates continuously.

Cell, discontinuous measuring: A measuring cell which operates intermittently and not necessarily at fixed time intervals.

Cell, sequential measuring: A measuring cell which operates according to a succession of operations on the sample or on the sensitive elements (or on both), these operations being carried out according to one or more repetitive programs.

Ceramic filter: A component of a stack sampling system which is suitable for high temperature use; also known as a ceramic thimble.

Chain reaction: Chain reactions are characterized by a series of interrelated steps involving initiation, propagation, and termination. If the propagation steps recur a number of times for each initiation step and termination step, the reaction is called a chain reaction. For example, in the Cl₂-H₂ system which is irradiated within the Cl₂ absorption band, the reaction is initiated by the photodissociation of Cl₂: Cl₂ + hn --> 2Cl. The propagation steps involve the reaction and reformation of Cl atoms: Cl + H₂ --> HCl + H; H + Cl₂ --> HCl + Cl; eventually termination occurs largely through the reassociation of Cl atoms: 2Cl + M --> Cl₂ + M. M is a third body which removes some of energy released by the formation of C1, and allows it to be stabilized. The C1,-H, system can have a chain length of up to a million at low light intensities; i.e., 106 molecules of HCI

can form for each Cl₂ molecule photodissociated. In the atmosphere the oxidation of NO to NO₂ which occurs in the sunlight-irradiated mixture of NO_x and hydrocarbons, a complex series of chain reactions occur involving HO-, HO₂-, and RO₂-radicals, NO, hydrocarbons, and their oxidation products. GB: In chain polymerization reactions, reactive intermediates of the same type, generated in successive steps or cycles of steps, differ in molecular mass, as in

RCH₂CHPh + H₂C=CHPh --> RCH₂CHPhCH₂CHPh

Charcoal: A carbonaceous solid residue which is formed in the destructive distillation of wood. It has a very high surface area per unit weight which makes it especially useful as an absorber of various gases and vapours.

Checkwork: The multiple openings in a refractory wail which promotes turbulent mixing of the products of combustion.

Chemical ionization: The process in which primary ions, commonly formed by electron impact, react with neutral reagent gas molecules (e.g., by electron or proton transfer) to farm reagent ions. The extent of fragmentation of the reagent ion can be controlled through the exothermicity of the transfer reaction. Commonly employed in chemical ionization mass spectrometry.

Chemical potential: (GB) The chemical potential m _B of a substance B in a mixture of substances B, C,..., is defined by

where G is the Gibbs energy of the mixture, T is the thermodynamic temperature, p is the pressure, and n_B, n_C,..., are the amounts of the substances B, C,..., in the mixture.

Chemisorption (or chemical adsorption): Adsorption which results from chemical bond formation (strong interaction) between the adsorbent and the adsorbate in a monolayer on the surface. GB: Adsorption in which the forces involved are valence forces of the same kind as those operating in the formation of chemical compounds. If the molecule is adsorbed without fragmentation, the adsorption process is non-dissociative. Adsorption of carbon monoxide is frequently of this type.

Chemiluminescent methods of detection (analysis): Methods which utilize gas or liquid phase reactions between two molecules which produces a third molecule in an excited state. The wavelength distribution of the light emission from the excited molecule is characteristic of the species; in some circumstances this may be used to identify and to determine the concentration of one of the reacting species.

Chemiluminescence analyser: Instrument consisting of a reaction chamber with separate inlets for the sample and reagent gas, an optical filter, a photomultiplier and a signal processing device. The reactive gas is introduced in excess. The quantity of light produced is proportional to the sample flow rate and the concentration of the measured substance in the sample under specified temperature and pressure conditions. The filter limits the wavelength to the region of interest and helps to eliminate interferences.

Chimney (stack): A vertical flue for conducting cooled combustion products of a process to the atmosphere. This term also includes structures and opening of any kind from or through which smoke, grit, dust or fumes may be emitted.

Chimney effect: A vertical movement of a localized mass of air or other gases which occurs due to local temperature differences.

Chimney height (stack height), effective: A height used for the purposes of calculating the dispersion of emitted gases from a chimney, and which differs from the real chimney height by an amount which depends on such factors as the exit velocity, buoyancy effects and wind speed; it may also be affected by the local topography. It denotes the maximum height of the centre of a plume path above the level of the ground. The effective height may be above or below the actual chimney height, although the former case is most common.

Chromatogram in chromatography: A graphical presentation of detector signal against time, having a baseline and peaks.

Chromatographic analysis (chromatography): A method of separation of different chemical species based upon selective adsorption. Gas chromatography involves the separation of components (gases or vapours) in a gaseous stream (carrier gas) on a column of some suitable sorbent (stationary phase). Nonvolatile components are often separated by liquid chromatography where a solution of the components is separated by partition between a liquid mobile phase and a "solid" stationary phase. In both cases, the stationary phase may either be a liquid (often chemically bonded to the support) in which the components selectively dissolve, or a solid on which they are selectively adsorbed. Ion chromatography and ion exchange chromatography are useful for the separation and analysis of the ions present in rain-water or cloud-water. GB: The stationary phase may be packed in a column, spread as a layer, or distributed as a film, etc,; in these definitions chromatographic bed is used as a general term to denote any of the different forms in which the stationary phase may be use. The mobile phase may be gaseous or liquid.

Cloud: An aerosol of the atmosphere which is dense enough to be perceptible to the eye. Usually refers to an assembly of water-droplet aerosols suspended in the atmosphere, although dust clouds are also recognized.

Clouds, altostratus: A grey, uniform, striated or fibrous sheet but without halo phenomena, and through which the sun is seen only as a diffuse, bright patch or not at all; usually at elevations 2 000-6 000 m; vertical velocities of 0.05-0.1 m s^-1.

Clouds, altocumulus: A dappled layer of patch or cloud composed of flattened globules that may be arranged in groups, lines or waves collectively known as billows; 2 000-6 000 m; vertical velocities of 0.05-0.1 m s^-1.

Clouds, cumulus: Detached, dense clouds with a dome-shaped upper surface with sharp-edged, rounded protuberances and a nearly horizontal base; 600-6 000 m or more; convective motion with rising, large bubbles of warm air with vertical speeds of 1-5 m s^-1.

Clouds, cumulonimbus: Heavy masses of dense cloud whose cumuliform summits rise in the forms of towers, the upper parts having a fibrous texture and often spreading out into the shape of an anvil; these clouds generally produce showers of rain and sometimes of snow, hail or soft hail, and often develop into thunderstorms; up to 12 000 m; summits may be as cold as -50 °C; strong convective motions with vertical velocities of 3 to greater than 30 m s^-1.

Clouds, nimbostratus: An amorphous, dark grey, rainy cloud layer reaching almost to the ground; 300-600 m: vertical velocities of 0.05-0.2 m s^-1.

Clouds, stratocumulus: A layer of patches composed of laminae or globular masses arranged in groups, lines, or waves and having a soft, grey appearance; very often the rolls are so close together that their edges join and give the undersurface a wavy character; the process of formation (cumulogenesis) involves the spreading out of the tops of cumulus clouds, the latter having disappeared; <2 000 m; usually warmer than -5 °C; vertical velocities usually <0.1 m s^-1.

Clouds, stratus: A uniform featureless layer of cloud resembling fog but not resting on the ground; when this very low layer is broken up into irregular shreds, it is designated as fractostratus; 300-600 m; widespread irregular stirring and lifting of the shallow layer of cool, damp air near the ground.

Coagulation (flocculation): Process of converting a finely divided or colloidally dispersed suspension of one substance (usually a solid) in a liquid into larger-size particles which do settle or precipitate under the influence of gravity. Aluminum sulfate or other salt which contains a trivalent or divalent metal ion is sometimes used for this purpose. For atmospheric aerosol, coagulation denotes the process by which particles grow due to the attachment of smaller, more mobile particles subject to Brownian motions. Aitken particles coagulate efficiently with larger particles. GB: When a sol is colloidally unstable (i.e. the rate of aggregation is not negligible) the formation of aggregates is called coagulation or flocculation.

Coal desulfurization: Processes by which sulfur compounds are removed from coal. Some of the sulfur content of coal is composed of iron pyrites (FeS) which has a density (about 5 g cm^-3) very different from that of coal (1.25-1.45 g cm^-3) and can be removed by mechanical cleaning processes. Many processes have been used to remove SO₂ from flue gases and others are under study: limestone scrubbing, catalytic oxidation, fluidized bed reactors, etc.

COH, coefficient of haze: One technique of measurement of the amount of filterable particulate matter suspended in air which has been used in the past depends upon drawing a measured sample of air (usually 1000 linear feet) through a paper or membrane filter. A measurement is made of the intensity of light transmitted through the dust spot formed relative to that transmitted through an identical clean filter. The dirtiness of the air is reported in terms of the COH unit. This relates to the quantity of particulate material which produces an optical density, lg(I_o/I) of 0.01 when measured by light transmission at 400 nm and relative to the transmission of an identical dust-free filter taken as 100%. Thus a filter which transmitted 50% relative to the clean filter has an absorbance of 0.301 or 30.1 COH units. This is not a recommended measure of filterable particulate matter since the size, color and other properties of the aerosol and the air in which it is suspended affect the results.

Collection efficiency: A term which characterizes an entire sampling and sample pretreatment procedure, usually represented as a percentage of the original amount of the analyte which is left for measurement (signal formation) after having passed through this procedure. This term also applies to the effciency of collection of an air pollutant by an arrestment plant. For particulates, the collection efficiency is size related.

Collector: A device for removing and retaining contaminants from air or other gases. Term which is often applied to cleaning devices in exhaust systems. Also used to designate a device for removing and retaining samples from media in different environmental compartments to be investigated. A collector is sometimes used to describe a scavenger. GB: A solid substance added to or formed within a solution to collect a micro- or macro-component.

Collision number (z) or collision frequency: The number of collisions between two molecules A and B which occur per unit volume per unit time when both A and B are at a concentration of one molecule per unit volume; z_AB = s _AB^2{ 8p kT(m_A + m_B)/m_Am_B} ^1/2, where s _AB = (s _A + s _B)/2, s _A and s _B are the diameters of A and B, respectively, assumed to be rigid spheres, k is the Boltzmann constant, and m_A and m_B are the masses of the molecules A and B, respectively. The number of A-B collisions cm^-3 s^-1 is given by z[A][B], where [A] and [B] are the concentration (molecules per unit volume) of A and B, respectively. Conventional units for z are number of collisions cm^-3 s^-1. GB: In simple collision theory the rate coefficient of a bimolecular reaction between A and B is expressed as

k = p_ABz_ABe^-E/RT

where p_AB the steric factor, allows for the fact that, even when the energy requirements are satisfied, all collisions are not effective in causing reaction.

Colorimeter: An instrument used for color measurement based on optical comparison with standard colors.

Combustion chamber: The region within which solids, vapours and gases from the primary chamber are burned and the some settling of fly ash takes place.

Combustion gases: Gases and vapours produced in furnaces, combustion chambers or in open burning.

Complementary gas: Generally the most abundant component (pure gas or gas mixture) which makes up the mixture intended for the calibration.

Concentration, gas and liquid solutions: This represents the quantity of matter (mol, g, molecules, or other) per unit of volume (cm³, L, m³, or other). The IUPAC GB designation of this quantity is the amount of substance concentration (with the SI base unit of mol m^-1, but practical units of mol dm^-3 or mol L^-1). For a gas phase species it is common to describe the concentration as the number of molecules of the particular species per cm³ of air; this unit is most common in gas phase kinetics and in calculations involving chemical changes in the atmosphere. The number concentration or number density, often employed in physics and in physical chemistry, describes the number of molecules per cm³; however in writing the units for this quantity, molecules are understood but omitted; only cm" is written. The most common method of describing concentration of a given solute in the liquid phase (such as cloud-water) is the amount of the compound or ion (in moles) present per litre of the solution. One mole of a given species is 6.02213 x 10²³ (Avogadro constant) of these species. The mixing ratio is also employed as a measure of relative concentration. For a non-reactive gas, the mixing ratio does not change with altitude although the absolute concentration (molecules cm^-3) decreases. Thus one part-per-million by volume (1 ppmv) is a concentration equivalent to one molecule of a given species present in one million molecules (10⁶) of air. Similarly one part-per-billion by volume (1 ppbv) represents a molecular ratio of one molecule for every 10⁹ molecules of air; one part-per-trillion by volume (1 pptv) represents one molecule per 10¹² molecules of air. Concentrations of solids suspended in air are sometimes given in units of mass of the species per unit volume (e.g., micrograms per cubic meter of air). At times the mass fraction is used in contrast to the use of the volume, molecule, or mole fraction (e.g., ppmv); thus one part-per-million by mass (ppmm) is one unit of mass of a given species per million mass units of the mixture.

Concentration, ground level: The concentration of a chemical species, normally a pollutant, in air; usually measured at a specific height above the ground.

Concentration, particle: Commonly expressed in several ways: mass concentration (usually as m g m~3) or number concentration (number of particles cm^-3); modern instrumentation allows measurement of the number of particles as a function of size as well as the total number present in a given air volume. For atmospheric aerosols, this is a complex distribution for which diameters range from below 0.01 to above 100 m m; the particles making the highest contribution to the total number density are in the size range below 0.1 m m, those contributing most to the total surface area are in the 0.1 to 1.0 m m range, while those with the highest contribution to the volume or mass of the aerosol come from both the 0.1 to 1.0 and 1.0 to 100 m m ranges.

Condensation: The physical process of converting a material from a gaseous or vapour phase to a liquid or solid phase; this commonly results when the temperature is lowered and/or the vapour pressure of the material is increased. The tendency exists for condensation to occur when the partial pressure of a given component of a gaseous mixture at a given temperature exceeds the vapour pressure of the liquid or solid form of that component at the given temperature.

Condensation nuclei (sometimes symbolized as CN): A particle, either liquid or solid, or an ion upon which condensation of water vapour (or other substances) begins in the atmosphere. Condensation nuclei are usually very small hygroscopic aerosols (0.001 to 0.1 m m in diameter), but these are not as abundant as the smaller particles. The number of CN which are active (initiate condensation) in a given air mass may be a function of the relative humidity. Usually CN are counted as the active nuclei at about 300% relative humidity, while cloud condensation nuclei (CCN) are counted as the number of active nuclei at relative humidity less than or equal to 102%.

Constituent (with reference to an air sample): A component of the air sample for which a specified quantity is to be determined by measurement or analysis; see air quality characteristic.

Contaminant, air: A substance, gaseous material or aerosol, which is present in an air mass at levels greater than in clean air. An air contaminant has been added commonly by anthropogenic activity; see also air pollutant.

Continuous air monitoring program (CAMP): In the United States the CAMP program represents a series of monitoring stations which were established near large cities for the continuous monitoring of gaseous pollutants, SO₂, NO, NO₂, CO, hydrocarbons, and O₃.

Control agencies: Air pollution control agencies, established nationally or locally, which are designated to monitor and control air pollution in their designated areas through the enforcement of air pollution control legislation.

Controlled atmosphere: A synthetic gaseous sample of pure air which may contain carefully determined amounts of certain contaminants; this may be used as a standard for the calibration of analytical techniques, as a simulated environment for the study of biological responses, or for other purposes.

Convection (as applied to air motion): Vertical motion of the air induced by the expansion of the air heated by the earth's surface and its resulting buoyancy.

Correlation coefficient: A measure of the degree of interrelationship which exists between two measured quantities, x and y; the correlation coefficient (f) is defined by the following relation:

where x_i and y_i are the measured values in the ith experiment of n total experiments. and are the arithmetic means of x_i and y_i:

(similar expression for )

The linear correlation coefficient indicates the degree to which two quantities are linearly related. If x = ay is followed then r = 1, and departures from this relationship decrease r, if interpretations of data based on the linear correlation coefficient are to be made, one should consult a book on statistics.

Cracking: The thermal or catalytic decomposition of a compound such as a hydrocarbon into chemical species of smaller molecular weight.

Critical temperature: That temperature, characteristic of each gas, above which it is not possible to liquefy a given gas.

Critical point: The temperature and pressure at which the liquid and vapour intensive properties (density, heat capacity, etc.) become equal. It is the highest temperature (critical temperature) and pressure (critical pressure) at which both a gaseous and a liquid phase of a given compound can coexist.

Cryogenic: Term used to describe low temperature processes, apparatus, etc.; usually applied to systems operated at the temperature of liquid nitrogen, helium or other condensed gas which boils at a very low temperature (at atmospheric pressure); see sampling, cryogenic.

Cumulative sample: A sample collected over an extended period of time.

Cupola: A vertical shaft furnace used for melting metals; the melting of ores is accomplished in a blast furnace.

Cut off (as applied to aerosol sizes): The size of particles at which the retention efficiency of an instrument device drops below a specified value under defined conditions.

Cyclone (collector): A dust, grit, or droplet separator utilizing essentially the centrifugal force derived from the motion of the gas. The flow of gases containing suspended particles into the device is transformed into a confined vortex from which centrifugal forces tend to drive the suspended particles to the wall of the cyclone body. The agglomerated particles are subsequently removed from the cyclone by gravitational action.

Cyclone (meteorology): A large circulatory wind system around a region of low atmospheric pressure; rotation is counterclockwise (viewed from above) in the northern hemisphere and clockwise in the southern hemisphere.

Dark reaction (darkness reaction): A chemical reaction that does not require or depend on the presence of light. Contrasts with a photochemical reaction which is initiated by light absorption by one or more of the reactants.

Dead time: see Analyser, dead time of.

Decay rate: The rate at which a pollutant is removed from the atmosphere either by reaction with reactive transient species such as the HO-radical, O₃, etc., by photodecomposition initiated by light absorption by the impurity, or by loss at the surface of aerosols, the earth, etc. The decay rate as applied to radioactive materials is related to the radioactive half-life (t_1/2) of the particular isotopic species A and its concentration [A]_t at the given time (t): Rate = [A]_t(ln 2)/t_1/2.

Deliquescence: The process that occurs when the vapour pressure of the saturated aqueous solution of a substance is less than the vapour pressure of water in the ambient air. When water vapour is collected by the pure solid compound, a mixture of the solid and liquid, or an aqueous solution of the compound forms until the substance is dissolved and is in equilibrium with its environment; at this time the vapour pressure of water over the aqueous solution will equal the partial pressure of water in the atmosphere in contact with it. A crystalline salt aerosol particle will deliquesce in the atmosphere when the relative humidity surpasses a characteristic value, the so-called deliquescence point.

Demister: Apparatus made of wire mesh or glass fibre which is used to help remove acid mist as in the manufacture of sulfuric acid. Demisters are also components of wet arrestment plants.

Density: The mass per unit volume of a gas, liquid, or solid under specified conditions (temperature, pressure, etc.).

Density, number: The number of atoms, molecules or aerosol particles per unit volume of gas (commonly in units of molecules cm^-3).

Denuder system (tube or assembly): An apparatus used to separate gases and aerosols (over a given diameter) which is based upon the difference in diffusion velocity between gases and aerosol particles. Usually a tube containing a selective internal wall coating which removes the gaseous compounds at the wall.

Deodorizer: Equipment for the removal of noxious gases and odors, which may consist of combustion, absorption, or adsorption units; see emission control equipment.

Deposition: Deposition is normally considered to be one of two types: dry deposition is the process by which aerosols and gases in the air are deposited on the surface of the earth (soil, water, rock, plants, etc.); this is termed "dry" deposition even when the receptor surface is moist. Wet deposition is that process which involves the transport of chemicals to the surface of the earth by water droplets or snow crystals which scavenge pollutants as they form and fall through the atmosphere.

Deposition velocity: The ratio of flux density (often given in units of g cm^-2 s^-1) of a substance at a sink surface to its concentration in the atmosphere (corresponding units of g cm^-3). While the units of this ratio are clearly those of velocity (in this case cm s^-1), the ratio is not a flow velocity in the normal sense of the word.

Desorption: The removal of a substance from a surface on which it has become absorbed. Desorption is accelerated by heating and exposure of the surface to a low pressure (or vacuum). GB: The converse of adsorption, i.e. the decrease in the amount of adsorbed substance.

Desulfurization: The process by which sulfur is removed from a material such as coal or oil. It may involve one of many techniques including elutriation, froth flotation, laundering, magnetic separation, chemical treatment, etc.

Detection: The process of establishing the presence of air pollutants by means of specific instrumentation and sampling.

Detection limit, lower: The minimum concentration of a compound in an air sample which can be determined by an analytical method with a given statistical probability. Usually the lower detection limit is defined as three-times the standard deviation of the noise of an analytical method under the assumption that its distribution is Gaussian. In this case a concentration can be detected with a probability of 99.7%. This is sometimes referred to as the absolute detection limit (GB).

Detection limit, relative: GB: Smallest amount of material detectable (with 99.7% probability) in a matrix relative to the amount of material analysed - given in atomic, mole or weight fractions.

Detection limit (threshold): The minimum concentration of substance which produces an observable response.

Detector: An instrument or part of an instrument which indicates the presence of air quality characteristic by means of some specific spectroscopic or chemical property of the pollutant.

Dew point: That temperature at which water vapour present in the atmosphere is just sufficient to saturate it. When air is cooled below the dew point, the excess of water vapour appears as tiny droplets or crystals of ice depending on the temperature of the air mass. This term is also applicable to acid gases as in the problem of acid corrosion and acid smut formation.

Diameter, equivalent: The diameter of a spherical particle which will give identical geometric, optical, electrical or aerodynamic behavior to that of the particle (non-spherical) being examined; sometimes referred to as the Stokes diameter for particles in non-turbulent flows.

Diffraction: A modification which light (or electron, neutron beams, etc.) undergoes in passing by the edges of opaque bodies or through narrow slits or in being reflected from ruled surfaces (or crystalline materials). The light waves, owing to their wave-like nature appear to be deflected and produce fringes of parallel light and dark bands corresponding to regions of constructive reinforcement or destructive interference, respectively, of the waves.

Diffraction analysis: The application of diffraction techniques (X-rays, electrons, neutrons) which are sometimes used to identify the presence of certain solid aerosols and dust particles through the characteristic diffraction patterns which result from each unique crystal structure.

Diffuser: A porous plate or tube, commonly made of carborundum, alundum, or silica sand, through which air is forced and divided into minute bubbles for diffusion in liquids.

Glossary of atmospheric chemistry terms (Recommendations 7990)

Diffusion: The spreading or scattering of a gaseous or liquid material. Eddy diffusion in the atmosphere is the process of transport of gases due to turbulent mixing in the presence of a composition gradient. Molecular diffusion is the net transport of molecules which results from their molecular motions alone in the absence of turbulent mixing; it occurs when the concentration gradient of a particular gas in a mixture differs from its equilibrium value. Eddy diffusion is the most important mixing process in the lower atmosphere, while molecular diffusion becomes significant at the lower pressures of the upper atmosphere.

Diffusion battery: An aerosol sizing instrument for particles with diameters below 0.2 m m. The fractionation is based on different diffusivities of the small particles and their deposition on the walls of the long parallel or circular channels, formed by equally spaced plates, bundles of small bore parallel tubes or sets of stainless wire screens.

Diluent gas: A gas of known quality introduced for analytical purposes so that it quantitatively lowers the concentration of the components of a gaseous sample; this may also be the complimentary gas.

Dioxin: Colloquial (short) name of a toxic by-product (and sometimes contaminant) of herbicides; the full name of this species is: 2,3,7,8-tetrachlorodibenzo[b,e][l,4]dioxin.

Discomfort threshold: The lowest value (e.g., concentration of an impurity, etc.) at which a sensation of discomfort is perceived; a measure which varies from person to person.

Dispersion: The dilution of a pollutant by spreading in the atmosphere due to diffusion or turbulent action (eddy diffusion).

Dissociation energy: see bond dissociation energy.

Diurnal variation: Indicates variations which follow a distinctive pattern which recurs with a daily cycle.

Dobson unit: Unit sometimes used in the description of the total ozone in a column of air overhead. It is given as the thickness (in units of 10^-3 cm or 10^-5 m) of that layer which would be formed by the total ozone in a column reduced to the pressure of 760 Torr and temperature of 0 °C.

Dosage: As applied to an air pollutant in an exposure chamber, dosage is commonly defined as the concentration of the pollutant times the duration of exposure.

Downwash: As applied to the action of chimney gases, it is the downward motion of the chimney gases brought on by eddies which form in the lee of a chimney when the wind is blowing. It may result in bringing the flue gases to the ground prematurely.

Droplet: A small liquid particle. The size of droplets encountered in the atmosphere extends over a wide range; e.g., liquid aerosol solutions which make up the fine particle fraction of continental tropospheric aerosol are usually <2 m m in diameter. Cloud water droplets usually have diameters in the range of 5 to 70 m m, while rain droplets commonly have diameters ranging from 0.1 to 3 mm.

Dry bulb temperature: In psychrometry, the temperature of the gas measured by a dry bulb thermometer; see psychrometry.

Drying agents: Substances which remove water (liquid or gaseous).

Dust: Small, dry, solid particles projected into the air by natural forces, such as wind, volcanic eruption, and by mechanical or manmade processes such as crushing grinding milling, drilling, demolition, shovelling, conveying, screening, bagging, and sweeping. Dust particles are usually in the size range from about 1 to 100 m m in diameter, and they settle slowly under the influence of gravity.

Dust collector: A device for monitoring dust emissions. Also the equipment used to remove and collect dust from process exhaust gases; this may employ simply sedimentation (dustfall jars, coated slides, papers, etc.), inertial separation (cyclones, impactors, impingers, etc.), precipitation (thermal and electrostatic) or filtration.

Dust fall: Solid particles in the air which fall to the ground under the influence of gravity.

Dynamic range of an analyser: The ratio between the maximum usable indication and the minimum usable indication (detection limit). A distinction may be made between the linear dynamic range, where the response is directly proportional to concentration, and the dynamic range where the response may be non-linear, especially at higher concentrations.

Eductor: A liquid pump for mixing air with water which operates under a jet principle using the liquid under pressure as the operating medium.

Eddy: In turbulent fluid motion, a blob of the fluid that has some definitive character and moves in some way differently from the main now.

Eddy dispersion (diffusion): The process by which substances are mixed in the atmosphere or in any fluid system due to eddy motion.

Efflorescence: The reverse of deliquescence: the drying of a salt solution when the vapour pressure of water in the saturated solution of a substance is greater than the partial pressure of water in the ambient air. Also refers to the loss of water of crystallization from a solid salt such as Na₂CO₃·10H₂O.

Effluent: Any spent liquors or other waste material which are emitted by a source (waste from plating shops, pickling tanks, sewage treatment plants, chemical manufacturing plants, etc.).

Einstein unit: 1 mole (6.02213 x 10²³) of quanta or photons of electromagnetic radiation; for monochromatic light of wavelength l , a light beam of energy E (J s^-1) striking a surface, will have E/(6.022 x 10²³ x hc/l ) Einstein striking per sec; h is the Planck constant (6.626 x 10^-34 J s) and c is the velocity of light (2.998 x 10⁸ m s^-1).

Electrochemical methods of detection (analysis): Methods in which either current or potential is measured during an electrochemical reaction. The gas or liquid containing the trace impurity to be analysed is sent through an electrochemical cell containing a liquid or solid electrolyte and in which an electrochemical reaction specific to the impurity takes place.

Electrochemical methods, amperometric detection: The current is proportional to the concentration of the species generating the current.

Electrochemical methods, coulometric detection: The current is directly proportional to the flow rate of the substance involved in the electrochemical reaction, and the amount of charge which flows is proportional to the amount of substances taking part in the reaction.

Electrochemical methods, potentiometric detection: The potential of a cell is related to the concentration (activity) of a reactant which is a component of the cell fluid.

Electron charge: The negative electric charge which appears on the electron or univalent ions [1.602 x 10^-19 coulombs or 4.803 x 10^-10 electrostatic units (esu)].

Electrostatic filter: Filters for which an electrostatic charge is applied to the filter element. A fibrous filter material is often pleated in between V-shaped supports consisting of electrostatically charged, metal rods which are insulated from the supporting frame.

Electrostatic precipitator: A device which separates particles from a gas stream by passing the carrier gas between pairs of electrodes across which a unidirectional, high-voltage potential is placed. The particles are charged before passing through the field and migrate to an oppositely charged electrode. These devices are very efficient collectors of small particles, and their use in removing particles from power plant plumes and in other industrial applications is widespread.

Eluate: GB: The effluent from a chromatographic bed emerging when elution is carried out.

Elute: To remove sorbed materials from a sorbent by means of a fluid called the eluent. GB: To chromatograph by elution chromatography. This term is preferred to the term develop, which has been used in paper chromatography and in thin-layer chromatography. The process of elution may continue until the components have left the chromatographic bed.

Elutriation: The process of separating the lighter particles of a powder from the heavier ones by means of an upward directed stream of fluid (gas or liquid).

Emission: The total rate at which a solid, liquid, or gaseous pollutant is emitted into the atmosphere from a given source; usually expressed as mass per unit time. Primary emissions are those substances which are emitted directly to the atmosphere (e.g., NO, SO₂, etc.), while secondary emissions are formed from the primary emissions through thermal or photochemical reactions (e.g., ozone, aldehydes, ketones, sulfuric acid, nitric acid, etc.). The point or area from which the discharge takes place is called the source; the area in which the emission or its transformed products (e.g., in the case of aerosols, acidic deposition, etc.) may be deposited is called the receptor area or sink. Emission may be applied to noise, heat, etc., as well as pollutants.

Emission control equipment: Air pollution control equipment which either converts the pollutant chemically to a non-polluting substance or collects the pollutant by some means including gravity settling chambers, inertial separators, cyclonic separators, filters, electrical precipitators, scrubbers, (spray towers, jet scrubbers, Venturi scrubbers, inertial scrubbers, mechanical scrubbers and packed scrubbers). Certain gases and odoriferous compounds are controlled by combustion, absorption (spray chambers, mechanical contactors, bubble cap or sieve plate contactors, and packed towers), and adsorption units (packed beds or fluidized beds).

Emission flux: The emission per unit area of the appropriate surface of an emitting source.

Emission inventory: A systematic collection of information concerning the air pollution emissions in a given area. Usually the types of sources (power plants, refineries, etc.) and the height at which the discharge takes place as well as the source contribution (composition and rate of discharge) are summarized. The variation of these emissions with time of day, and month of the year are desired information in inventories as well.

Emission source: Several types of emission sources exist: Point sources of emission include: power plants, incinerators, refineries, steel mills, chemical plants, etc. Often the emissions for these sources are released from elevated stacks. Mobile sources of emission include: automobiles, trucks, buses, ships, airplanes, etc.

Enforcement: The act of preventing the discharge into the atmosphere from any source operating without a permit or in violation of the terms of a permit or rules, regulations, or orders of an pollution control agency.

Enthalpy: The enthalpy (H) of a system is defined by the equation

H = U + pV

where U is the internal energy, p is the pressure, and V is the volume of the system.

Entrainment: The act of forming a mist or fog droplets of a liquid carried off by the vapours of a boiling liquid or from a liquid through which bubbles of gas or vapour are passing rapidly.

Entropy: The entropy (S) of a system is defined by the equation

dS ³ dq/T

where dq is the infinitesimal heat absorbed by the system which is at the thermodynamic temperature, T.

Equilibrium: A dynamic or static state of balance between opposing forces or actions. A dynamic equilibrium exists between two different phases of a given pure compound (liquid-gas, gas-solid, solid-liquid) when there is no net change in the amount of either phase when the two remain in contact; this occurs at a given temperature when the vapour pressures of each of the two phases of the compound are equal. A dynamic equilibrium exists in a reversible chemical reaction system at a given temperature and pressure, when the reactions generating and destroying each reactant and product continue to occur, but there is no net change in the amounts of the reactants or products present.

Equilibrium constant: The constant (at a given temperature) that relates the concentrations of reactants and products in a reversible chemical reaction at equilibrium. Consider an equilibrium established in a reversible chemical system containing molecules of A, B, C, and D, which react by the stoichiometry: aA + bB cC + dD (where a, b, c, and d are the coefficients of the respective reactants and products in the balanced chemical equation describing the change). When the system is at equilibrium, the rates of the forward and reverse reactions become equal, and no further change in reactant and product concentrations occurs at a fixed temperature and pressure. At this time the equilibrium constant, K_p, is given by: K_p = [p_C]^c[p_D]^d/[p_A]^a[p_B]^b where p_X represents the pressure of the X species at equilibrium. Pressures must be replaced by fugacities for systems at high pressures. Equilibrium constants based upon fugacities are symbolized by K_f. For solution phase reactions, concentration units (or activities for concentrated solutions) are employed for the reactant and products, and a similar equilibrium expression can be written: K_c = [C]^c[D]^d/[A]^a[B]^b. When fugacities or activities are employed, the eguilibrium expressions hold accurately regardless of the starting pressures or concentrations of the reactants and products. For systems at reasonably low pressures (near one atmosphere) or low concentrations, the K_f and K_c expressions given above hold reasonably well.

Error, random: The random fluctuations observed in the output from a measurement apparatus or method when the input to the instrument or method is held constant.

Error, systematic: Errors in a measurement which stay more or less constant and which may be attributed to the particular design of the measurement. Such errors may in principle be accounted for and eliminated, in contrast to random errors, which may not be eliminated.

Evaporation: The physical process by which a liquid substance is converted to a gas or vapour. This may occur at or below the normal boiling point of the liquid (the temperature at which a liquid boils at 1 atmosphere pressure), and the process is endothermic.

Excitance, radiant, spectral: The derivative of the incident radiant power with respect to the projected area of a specified emitting surface (W cm^-2) With the adjective spectral, differentiation with respect to wavelength is also implied (W cm^-2 nm^-1). Irradiance and excitance are similar but refer to energy arriving at versus leaving from a surface, respectively.

Explosivity limits (or explosion limits); flammable limits: The concentration limits, usually of a substance in air, between which combustion will be self-sustaining.

Exposure: Subjecting a person, animal, plant, or material to an environment containing a significant concentration of an air pollutant.

Exposure chamber: In environmental studies, this is a suitable room or chamber in which the atmosphere may be controlled (relative humidity, pollutant concentrations, temperature, light intensity, etc.) and in which exposure to animals, plants, or materials can be made. The exposure of humans to small concentrations of any toxic or seemingly non-toxic chemical in chambers is a very hazardous practice which should be avoided; errors in the controls of the contaminant concentration and allergic reactions, etc., can occur with risk to life.

Extinction coefficient: see absorption coefficient, scattering cross section.

Eye irritation: Experienced by many persons upon ex